Show a dashed drop-area in empty kanban columns and the inbox

.kanban__cards:empty (shared by every status column and the sidebar's
inbox list) gets a dashed border, transparent background, and a
'Drop cards here' hint, echoing the dashboard's 'Create a project'
tile. Pure CSS: both draggables are already always-rendered even when
empty, so an empty one is a genuinely childless element and :empty
just tracks that -- it steps aside on its own once Sortable inserts a
drag-over ghost, and needs no JS empty-state flag.

Drops the sidebar's separate 'Nothing in the inbox.' message, now
redundant with the box itself. Verified the container really is
childless when empty via a DOM dump against a running instance (no
screenshot).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/README.md

@@ -78,6 +78,14 @@ project view is currently mounted (`route.name === 'project'`), that project's
 cards too -- either side of a drag could have been the inbox. A small form
 under the list adds a card straight to the inbox.
 
+Empty (no cards) is shown as a subtle dashed drop-area rather than blank
+space -- `.kanban__cards:empty` in `style.css`, so it's pure CSS keyed off the
+real DOM child count. Since a card's kanban column/inbox `<draggable>` is
+always rendered even with nothing in it (see above), that container is
+genuinely childless when empty, so the rule applies with no extra markup or
+JS state; it steps aside automatically once Sortable inserts its drag-over
+ghost. The same rule covers every kanban status column too (below).
+
 ## Project detail
 
 `/projects/:id` shows one project. It renders on a **full-width** layout (the